[i18n-activity] How to space base characters where shiftRuby kicks in for group ruby?

r12a has just created a new issue for 
https://github.com/w3c/i18n-activity:

== How to space base characters where shiftRuby kicks in for group 
ruby? ==
10.2.36 tts:rubyOverflow
https://www.w3.org/TR/2016/WD-ttml2-20161117/#style-attribute-rubyOverflow


jLReq makes a distinction between (a) handling of annotation alongside
 a single base character, and (b) annotation alongside multiple base 
characters in the `shiftRuby` case. For (b), the base characters are 
spread evenly as in this diagram.

![img2_3_36](https://cloud.githubusercontent.com/assets/4839211/22987949/56468b9e-f3a8-11e6-9af0-f6ed06654062.png)

How would one apply that behaviour specifically to the ruby items 
where `shiftRuby` has been applied to prevent overflow?

Please view or discuss this issue at 
https://github.com/w3c/i18n-activity/issues/339 using your GitHub 
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